Technical advance of vacuum degassing process for a molten steel in recent years has facilitated the production of ultra low carbon steels, leading to an ever-increasing demand for ultra low carbon steel sheets having good workability.
It is well known that such ultra low carbon steel sheets generally contain at least one element selected from the group consisting of Ti and Nb. Ti and Nb exhibit a strong, attractive interaction with interstitial solid solution elements (C, N) in the steel to easily form carbonitrides, enabling a steel free from interstitial solid solution elements (IF steel: interstitial free steel) to be easily produced. IF steels are free from interstitial solid solution elements causative of strain aging and deteriorated workability and, hence, feature a non-aging property and very good workability. Further, the addition of Ti and Nb plays an important role, that is, it refines the diameter of grains, of a hot rolled steel sheet of an ultra low carbon steel, which are likely to be coarsened, and improves the deep drawability of a cold rolled, annealed steel sheet. However, ultra low carbon steels with Ti and Nb added thereto have the following problems. First of all, the production cost is high because the cost associated with the addition of expensive elements such as Ti and Nb is added to the cost of vacuum treatment for achieving ultra low carbon. Secondly, the absence of C and N in solid solution in product sheets results in drawing-induced embrittlement or disappearance of paint bake hardening property (BH property). Thirdly, the base metal and the spot weld zone have poor fatigue properties. The reason for this is that the strength of the material is low due to the nature of the ultra low carbon steel and, in addition, the microstructure of heat-affected zone in the spot welded area is coarsened to form a brittle area. Fourthly, Ti and Nb are strong oxide formers, and the formed oxides deteriorate the surface quality.
A large amount of research and development have been done with a view to solving the above problems of IF steels. One conceivable means for solving the above problems is to use as a base material an ultra low carbon steel with Ti and Nb not added thereto. this is because the use of a steel not containing Ti and Nb, as the base material naturally leads to the solution of the above first, second, and fourth problems. The adoption of such means is found, for example, in Japanese Unexamined Patent Publications (Kokai) No. 63-83230, No. 63-72830, No. 59-80724, No. 60-103129, No. 1-184251, No. 58-141355, and No. 6-93376. In all of the above publications, attention is drawn to the properties influencing the press moldability of an ultra low carbon steel sheet not containing Ti and Nb, such as r value and elongation, and the BH property and the fabrication embrittlement resistance.
Regarding the fatigue properties as the third problem, however, only a few studies have been made. Japanese Unexamined Patent Publication (Kokai) No. 63-317625 discloses a process for producing an ultra low carbon, cold rolled steel sheet excellent in fatigue properties of spot weld zone wherein Ti, Nb, and B are added in combination and the temper rolling is optimized. However, no mention is made of any method of improving fatigue properties in ultra low carbon steels free from Ti and Nb. Japanese Unexamined Patent Publications (Kokai) No. 6-81043, No. 6-81044, and No. 6-81080 disclose an ultra low carbon steel sheet, having excellent fatigue properties and deep drawability, containing at least one member selected from the group consisting of Ti and Nb, and a process for producing the same.
These unexamined publications disclose the method of increasing the yield strength and improving the fatigue properties of the base metal. However, no study, has been made on the fatigue properties of a joint in its spot weld zone. Further, for the above unexamined publications, only ultra low carbon steels with Ti and Nb added thereto are contemplated, and no study is made on ultra low carbon steels contemplated in the present invention, that is, those substantially free from Ti and Nb.
In general, for ultra low carbon steel sheets free from Ti and Nb, the fatigue properties of base metal are poor due to low yield strength, and heat applied at the time of spot welding is likely to cause abnormal grain growth, leading to a possibility that the fatigue properties of the joint in its spot weld zone become unsatisfactory. As described above, no technique for preventing these unfavorable phenomena has been proposed.